Thermal energy harvesting for wearable devices

According to conservation laws, the total energy of a system is conserved, with the possibility of transforming from one form to another. A classic example is the collision of two billiard balls with the consequential “energetic sound” and heat developed at the point of contact.

In any part of the Earth, at any point in our universe, there is a temperature gradient, and therefore, nothing prevents us from having the energy to power a device. A considerable amount of thermal energy is also available at the industrial level by exploiting the power of production devices during process control. The Seebeck effect controls the physical process to produce electricity. The process then converts thermal energy, a by-product of other forms of energy such as chemistry and mechanics, into an electrical signal with a specific power.

An interesting approach to use, in wearable systems, is related to energy harvesting technology by generating small electric currents which exploit thermal energy as the difference between two temperatures, that of the body and that of the external environment. Temperature differences can be seen everywhere, both in natural and human-made environments. These differences can be used to create thermoelectric energy

Seebeck effect

The temperature difference between the ends of a conductor creates a potential difference. Seebeck demonstrates its existence experimentally, thinking in a first approach to have discovered a new form of the magnetic field by observing the movement of a compass needle; in reality, without his knowledge, he had discovered a new effect (hence his name) in the form of electric voltage. The magnitude of this voltage is related to the material and the temperature difference (Th –Tc ) according to the following relation:

Where S is the Seebeck coefficient. The maximum power collected is given by the following relationship:

Where A is the section of the material, ρm the resistivity, l the length of the thermocouple, and with T the temperatures of the warm side (with subscript h ) and cold (with subscript c ) are indicated. In the design phase, it is necessary to monitor the temperatures of each side with suitable sensors, such as thermistors, which measure the temperature according to the variation of its resistance as can be seen in the following relation:

Where R (T ) is the temperature resistance where T is temperature, B is a constant and T0 and R0 are temperature and resistance at 25 o C (environment) respectively. The temperature coefficient is the variation of R0 as a function of temperature, which can be expressed as follows:

In the design of a thermogenerator, it is necessary to stabilize the temperature on both sides of the thermocouple. The temperature can also be controlled by means of PID controllers which, through the feedback network, compensate to stabilize the process. Heaters, chillers, and Thermo Electric Generators (TEG) are created using thermoelectric materials.

The ideal thermoelectric materials will have:

Low thermal conductivity

High electrical conductivity

A high coefficient of Seebeck

Our bodies are relatively warm considering that the internal body temperature is 37° C. Skin temperatures are typically in the range of 32o C For typical indoor air temperatures, an energy harvesting device attached to a person's skin offers ΔT up to 10o C. A thermogenerator example is a Peltier cell. The Peltier cell is a thermoelectric device made up of many PN junctions in series (Figure 1).

Figure 1

Peltier cell

Circuit solution

The conditioning circuits play a fundamental role in an energy harvesting system, through various parameters such as input impedance, and at the same time, circuit functions such as power control and filtering. The critical components include the transducer, whether it is a thermal, photovoltaic or vibrational source, as well as an IC energy conditioning circuit, a microcontroller and a storage device (supercapacitor).

The LTC3107 converter has been designed to meet this need by allowing you to easily extend battery life by adding an energy harvesting circuit to existing projects. By generating an output voltage that follows that of the main battery installed, it is possible to use the LTC3107 without any problem to obtain the possible cost containment with energy harvesting from free thermal energy sources, in new or pre-existing battery-powered systems. LTC3107, together with a small source of thermal energy, can prolong the life of the battery, in some cases up to its expiry, thus reducing the costs of periodic maintenance due to the replacement of the battery itself. It was designed to complete the battery or even power the load independently, according to the load conditions and the stored energy available (Figure 2).

Figure 2

System with wireless sensor equipped with battery and thermal energy harvesting circuit based on LTC3107

Another example is the LTC3331, an energy harvesting solution with full regulation that provides up to 50 mA of continuous output current. The LTC3331 integrates a wave rectification system, as well as a synchronous buck-boost DC / DC converter to create a single continuous output for energy harvesting applications, such as wireless sensor nodes (WSN) and various IoT devices (Figure 3). LTC3106 is an integrated buck-boost DC / DC converter optimized for multi-sources. In the absence of load, the LTC3106 consumes only 1.6μA when supplying an output voltage up to 5V.

Figure 3

Block diagram of the LTC3331

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